1. Field of the Invention
This invention relates to automatic radar altitude control of aircraft having vertical take-off and landing capabilities such as helicopters, and more particularly to automatic ground effect compensation to maintain altitude control of such craft subject to ground effect.
2. Description of the Prior Art
Ground effect can generally be defined as the reaction on the craft due to the downwash of air from the rotating blades striking the ground and then reflected back from the ground to the craft tending to increase air density and hence rotor lift as the craft enters the ground effect zone. The ground effect zone can be generally defined as a zone or volume having a diameter and height equal to the diameter of the rotor blades.
As the craft leaves the ground effect zone the ground effect decreases the lift capability of the sustaining rotor. Such a change in lift will normally be reflected in an increase in the power required to maintain the craft in a hover and/or altitude hold condition.
In existing helicopters having automatic radar altitude hold capability, the pilot may select through a conventional mode select panel or through manual override of the altitude hold, an altitude at which it is desirable to hover; for example, during a rescue mission. The autopilot through a servo amplifier drives a parallel trim actuator which is connected to the pilot's collective control stick through a preloaded force feel spring. Included also in the trim servo system is a clutch means, integral with the trim actuator, which allows the actuator control arm to be declutched from the servomotor. Also included is a detent switch operable by force spring and a collective control stick position sensor. A full disclosure and description of a typical helicopter trim actuator system useful in carrying out the present invention may be had by reference to U.S. Pat. No. 4,345,195 also assigned to the present assignee.
As is well known to those skilled in the art, by manipulation of the collective control stick against the force feel spring, the craft may be manually flown to the desired altitude and upon release of the collective control stick force, the autopilot will maintain the desired altitude. Typically, the desired altitude is a low altitude sensed by a radar altimeter. However, prior art radar altitude hold systems do not have the capability to automatically compensate for the change in lift effects encountered when the pilot maneuvers the craft into or out of the transition area at the upper limit of the ground effect zone. Without this capability, the pilot is required to exert a steady state force on the collective control stick to increase blade bite and a corresponding increase in engine power. If he releases the force, the stick will initially move toward its original trim position, thereby creating an undesirable transient in the aircraft's altitude due to the ground effect lift. To overcome this problem, some prior art systems provide a pilot operated collective force trim release switch on the collective control stick. With the addition of the trim release switch, as the craft enters the ground effect transition zone, for example, the pilot is required to depress the trim release switch in order to alleviate the force he is exerting upon the collective control stick. This results, however, in an abrupt discontinuity in the force feel characteristics of the system, and an unpleasant vertical jolt to the aircraft and its passengers.